1. Field of the Invention
The present invention relates to a sensor for gases, by means of which the gas is detected via its absorption in an infrared (IR) spectral range. A purpose of the invention is to manufacture such a sensor economically in large numbers and with improved characteristics.
2. Discussion of the Background
In accordance with their molecular structure, gases absorb radiation in the wavelength range from a few hundreds of nanometers up to a few micrometers (corresponding to the wave number range from a few 10.sup.4 cm.sup.-1 to a few 10.sup.3 cm.sup.-1). On the basis of its typical absorption spectrum the gas can be identified. This forms the basis of qualitative or quantitative spectral absorption analysis of gases and gas mixtures.
Spectral absorption analysis can be carried out by known spectral instruments designed for the IR range, in which the multi-frequency radiation is dispersed by means of a prism or a transmission grating or a reflection grating. These instruments can be used to address a very wide range of complex problems and to obtain accurate results. However, the known instruments are of considerable size and they can generally be used only in a fixed position and are comparatively expensive. Tuning through a complete spectrum as a rule requires long measuring times. They demand careful treatment and handling and as a rule can be operated only by skilled personnel. Consequently, permanent monitoring, for example, of a gas mixture in terms of a given component which may occur only sporadically, by means of one of the known dispersing systems, has hitherto been restricted to special cases.
The detection of a specific gas as a rule only requires a narrow wavelength window to be filtered out, which can be achieved with the aid of an interference filter. Sensors of this type, however, can only be used to detect that particular gas. A reference wavelength for eliminating external effects cannot be used. The manufacture of narrow-band interference filters is very laborious. The characteristic curve of such filters is strongly temperature-dependent.
Oxidizable gases, e.g. natural gas, are detected with the aid of sensors in which oxygen is abstracted from a metal oxide semiconductor layer, so that the conductivity of the semiconductor layer increases. These sensors react identically to various simultaneously present oxidizable gases, i.e. they are not gas-specific. They show long-term drift and are regenerated at regular intervals by treatment with oxygen. Similar behavior is shown by pellistors, which consist of a ceramic support coated with a catalyst.
In the case of electromagnetic sensors, the gas penetrating the sensors forms ions, and as a result the sensor voltage changes. While these sensors have sufficient selectivity, they can be used only for special gases. They can be employed in a limited temperature range, and their useful life is restricted to a few years.